THE TECTONIC SETTING AND SOURCE OF THE COLUMBIA RIVER BASALT GROUP (CRBG)

Recent 40Ar/39Ar dates from three laboratories (Berkeley, Open University, Oregon State) confirm that the CRBG eruption began at Steens Mt. at 16.6 Ma. Steens Basalt was followed conformably by Imnaha and then Grande Ronde Basalt, both formations increasing in volume towards the north between 16.6 and 15.0 Ma.

The eruption occurred in a tectonic setting of east-west extension and north-south compression. Extension increased from north to south, sometimes abruptly across Lawrence’s WNW-ESE right-lateral faults (OWL, Brothers Fault Zone etc.). E-W extension along the WNW-ESE fault zones created narrow graben or pull-apart structures (Western Snake River Plain) in addition to north-south graben between normal faults (La Grande graben). All graben are typically crossed by NW trending right-lateral faults that permit shear zones to occur without rotation.

Within this tectonic frame-work the orientation of faults, folds, feeder dikes and graben across the Columbia Plateau may be explained by the change in relative values of deviatoric stresses with depth. Using sigma H as the maximum horizontal stress (NNW-SSE), h as the minimum horizontal stress (WSW-ENE) and v as the vertical stress, then at the base of the crust óv=óH>óh to create the NNW-SSE tensional fissures used by the rising CRBG magma to reach the surface. In mid crust óH>óv>óh, to form the WNW right-lateral (OWL) and NNE left-lateral (Hite, Limekiln) conjugate strike-slip faults. Near the surface óH>óh>óv to create approximately east-west trending folds (Yakima folds, Lewiston structure, Blue Mts. anticline), often associated with steep reverse faults.

The massive and short lived tholeiitic CRBG eruptions were superimposed on smaller, sporadic, calc-alkaline eruptions associated with east-west extension from British Columbia to Nevada from at least the Eocene to the present (Pasco Basin, Republic Graben). The presence of the Yellowstone hot spot beneath SE Oregon at 16.6 Ma, the iron-rich nature of the tholeiitic magma, its exceptionally large volume and rate of eruption (220,000 km3 in 1.6 m.y. ), and restricted geographic location, all suggest a mantle plume as the most acceptable source model.